Advanced performance monitoring in all-optical networks.

This thesis investigates advanced optical performance monitoring approaches for future all-optical networks using the synchronous sampling technique. This allows for improved signal quality estimation, fault management and resource allocation through improved control of transmission at the physical layer level. Because of the increased transparency in next generation networks, it is not possible to verify the quality of the signal at each node because of the limited number of optical-electrical-optical conversions, and therefore new non-intrusive mechanisms to achieve signal quality monitoring are needed. The synchronous sampling technique can be deployed to estimate the bit error rate, considered an important quality measure, and hence can be utilised to certify service level agreements between operators and customers. This method also has fault identification capabilities by analysing the shapes of the obtained histograms. Each impairment affects the histogram in a specific way, giving it a unique shape that can be used for root cause analysis. However, chromatic dispersion and polarisation mode dispersion (PMD) can have similar signatures on the histograms obtained at decision times. A novel technique to unambiguously discriminate between these two sources of degradation is proposed in this work. It consists of varying the decision times so that sampling also occurs at both edges of the eye diagram. This approach is referred to as three-section eye sampling technique. In addition, it is shown that this method can be used to accurately assess first order polarisation mode dispersion and can simultaneously estimate the differential group delay (DGD) and the power splitting ratio between the two states of polarisation. Since synchronous sampling is employed, the effect of PMD on the sampling times is also investigated. For the first time, closed form relationship between the shift in sampling time, the DGD and the power splitting ratio between the polarisation states is obtained. Three types of high-Q filter based clock recovery circuits are considered: without pre-processing circuits that can be used for RZ format and with an edge detector or a squarer pre-processing circuits suitable for NRZ format. Moreover, this technique can be used to monitor chromatic dispersion and a large monitoring range of more than 1750ps/nm is experimentally demonstrated at 10Gbit/s. Since it can monitor PMD and dispersion, this method can be deployed to control dynamic PMD or dispersion compensators. Furthermore, this technique offers easy and quick inline eye mask testing and timing jitter assessment

Investigation of the use of electronic pre-distortion and MLSE equalization in long-haul transmission

The performance of an 11 Gbit/s NRZ transmission system over 1600km utilizing electronic pre-distortion coupled with 2sample/bit 32-state MLSE equalization is studied. We show that MLSE reduces the OSNR penalty in the linear and nonlinear regime

MLSE-EDC versus optical dispersion compensation in asingle-channel SPM-limited 800 km link at 10 Gbit/s

We investigate experimentally the effectiveness of MLSE-EDC for application with signals distorted by self-phase modulation, and compare the results with those obtained using a variety of optical dispersion compensation maps

Robust long-haul transmission utilizing electronic precompensation and MLSE equalization

We assess the performance of transmission systems utilizing electronic predistortion and MLSE equalization. In 11-Gb/s transmission over 1040km of standard fiber, the required accuracy of the EPD can be relaxed to +/-1400ps/nm using an 8-state Viterbi algorithm. © 2006 Optical Society of America

Long-haul 10 Gbit/s linear and non-linear IMDD transmission over uncompensated standard fiber using a SQRT-metric MLSE receiver

We experimentally demonstrated Intensity-Modulated Direct-Detection (IMDD) single-channel 1,040 km linear transmission and 800 km non-linear transmission at 10 Gb/s over standard single-mode (G.652) fiber, without any optical dispersion compensation or mitigation, using a Maximum-Likelihood Sequence-Estimation (MLSE) receiver employing the square-root (SQRT) branch metric with off-line processing. These experiments were designed as to probe the limits of the MLSE approach. They successfully showed that long-haul uncompensated transmission is in principle possible with MLSE, even in the presence of large uncompensated dispersion and strong intra-channel fiber non-linearities, provided that enough complexity can be built into the receiver. In the linear 1,040 km experiment, a Bit Error Rate (BER) of 10-3 was achieved with an Optical Signal-to-Noise Ratio (OSNR) penalty with respect to back-to-back of 2.9 dB, using two samples per bit and 16,384 trellis states. Several other set-ups were tested as well, including the use of only one sample per bit and fewer trellis states. In the non-linear 800 km experiment, power was ramped up to 12 dBm, exciting substantial Kerr non-linearity, whose induced spectral-broadening exacerbated the effects of the large uncompensated dispersion of the link. Using an MLSE receiver with 1,024 states, we demonstrated a non-linear threshold of 9 dBm. We benchmarked this experiment towards simulations addressing various electrical and optical dispersion compensation strategies. We also carried out an analysis of error run-lengths, on both experiments, which showed that error burstiness may change considerably depending on the number of processor states, OSNR and the amount of non-linearity in the link. © 2008 Optical Society of America

IMDD Transmission over 1,040 km of standard single-mode fiber at 10 Gbit/s using a one-sample-per-bit reduced-complexity MLSE Receiver

We demonstrate 1,040km NRZ-IMDD transmission at 10Gbit/s over G.652 fibre, without any optical dispersion compensation, using a reduced-complexity MLSE receiver employing, for the first time at this distance, one sample per bit only. © 2006 Optical Society of America

Experimental demonstration of real-time DSP with FPGA-based optical transmitter

Digital signal processing has the potential to overcome cost and power challenges associated with optical links. The questions to address for short links (avoiding the requirement for temperature control, laser patterning, differential mode delay in multimode fibres, increasing capacity at low cost) are different to those for ultra-long telecom links (mainly chromatic and polarization mode dispersion, and fibre nonlinearity), but both may be solved through the use of DSP. We have completed the construction and begun experimental tests of the first FPGA-based optical transmitter, based on the Xilinx Virtex 4 field programmable gate array chip and D/A converters, operating at 21.4 GSa/s. This will allow DSP for both short optical interconnects and long-haul transmission systems to be experimentally investigated. This paper reviews recent development work on this transmitter and presents initial results. © 2008 IEEE